The present invention relates to methods for dynamically selecting channels in a cellular communication system. More specifically, the present invention relates to methods for dynamically selecting channels in which received signal strength indication measurements are periodically taken on each channel when the channel is not in use and an estimated expected relative probability of non-interference of the measured channels is determined based on the received signal strength indication measurements.
In conventional cellular communication systems (hereinafter sometimes referred to for convenience as "macrocells", or as "existing cellular systems"), the voice channels used for the various cells in the system are selected and controlled manually. Existing cellular systems are not static; they evolve. For example, new cells are added, existing cells are sectorized, the transmit power level of cells are changed, and more channels are added to select cells. Channel plans for entire systems may have to be adapted to accommodate these changes. This might happen as often as once per month in some systems.
Some conventional cellular communication systems may employ mechanisms to detect interference on given channels. If that occurs, channel equipment is shut down, and the interference noted. Most conventional cellular communication systems employ fixed tuned combiners, which must be manually tuned. However, even self-tuning combiners impose severe restrictions on the channel combinations that can be used. Therefore, conventional cellular communication systems typically do not have the capability of automatically selecting new channels on which to operate.
Cellular Auxiliary Personal Communication Systems ("CAPCS") are deployed into this dynamic, changing environment to provide "personal" service to CAPCS subscribers within buildings. A typical CAPCS comprises a CAPCS transceiver, a CAPCS base station connected to a PBX in a building and a plurality of CAPCS subscriber units. The PBX is generally connected to the public switched telephone network. Thus, the CAPCS base station provides the interface between a CAPCS subscriber unit and the building's conventional telephone system.
CAPCS generally use the same frequency band as conventional cellular communication systems. Because CAFCS subscribers in a building are located relatively close to the CAPCS transceiver, CAPCS can operate at lower power levels than conventional cellular communication systems. The lower power levels also help insure that the CAPCS does not interfere with surrounding conventional cellular communication systems.
The deployment of numerous such systems, can result in combinations too complex for effective manual channel planning. Even if manual planning could be performed, the increase in the equipment that would have to be manually changed makes that task more and more difficult.
Because the CAPCS transmits at lower power levels and employs linear combiners, the controlling mechanism for a CAPCS can easily change the transmit frequencies. Also, with available broadband combiners, the transmit frequencies can be spaced closer than those in conventional cellular communication systems.
Generally, manufacturers of CAPCS employ some mechanism to detect channel interference and select a new channel. One example of such system is described by John Avery in a May, 1995 article published in the trade magazine "Microwaves & RF". The system described by Avery seems to be concerned with establishing the daily pattern of activity that might be expected on a macrocell channel, (a channel used by an existing conventional cellular system), and then selecting channels for the CAPCS based on this pattern of activity to avoid interference. Apparently, this system presumes that there is correlation between probability that a macrocell channel will be used and the time of day at which it will be used.
This might be true in some installations. For example, if the CAPCS "views" a macrocell that serves a freeway, it may be that the channels used by the macrocell that cover the freeway are more available during non-rush hours. However, for this to be true, the CAPCS must presume that channels used by other macrocells are available during rush hours.
Accordingly a need arises for an improved method for selecting channels in a CAPCS.